1. Field of the Invention
The present invention is related to a fixing device and an image forming apparatus including the fixing device, such as a copier, a printer, or the like using an electro-photographic technology.
2. Description of the Related Art
Conventionally, for example, in an image forming apparatus using an electro-photographic scheme, a toner image is formed on an image carrier, and the toner image is transferred onto a recording medium. After the toner image is transferred, the recording medium is carried to a fixing device, and is heated and pressed by the fixing device, so as to fix the toner image on the recording medium and then eject the recording medium.
In general, as illustrated in FIG. 1, in the fixing device, a fixing belt 52 is set around a fixing roller 53 and a heating roller 54 internally having a heat source 55, and the fixing belt 52 is clipped and pressed by the fixing roller 53 and a pressure roller 51. A paper sheet (recording medium) 59 is passed through between nip parts of the fixing roller 53 and the pressure roller 51. An entry guide 57 is provided to guide the paper sheet 59 to the nip parts at an upstream side above the nip parts formed by the fixing roller 53 and the pressure roller 51, and an output guide 58 for guiding the paper sheet 59 from the nip parts to be ejected is provided at a downstream side. That is, the fixing roller 53 rotates in a direction of an arrow A1 and the fixing belt 52 moves in a direction of an arrow A2. In this case, the pressure roller 51 contacting and pressing the fixing roller 53 through the fixing belt 52 rotates in a direction of an arrow A3. The paper sheet 59 passes through the nip parts by being guided by the entry guide 57 in accordance with a direction of an arrow B, and is carried out from the nip parts by being guided by the output guide 58.
In the fixing device, it is required to assure a capability of stably fixing an image while maintaining a target heating temperature. Conventionally, a technology has been used to control turning on and off the heat source (heater) 55 for heating the heating roller 54 based on a temperature of the fixing belt 52 detected by a temperature detecting part 56. In detail, in a case in which a temperature detected by the temperature detecting part 56 is lower than a target control temperature being a predetermined value, a duty of turning on electricity for the heat source (heater) 55 is set to be 100% and the heat source (heater) 55 is turned on. In a case in which the temperature detected by the temperature detecting part 56 is higher than the predetermined value, the duty of turning on electricity for the heat source (heater) 55 is set to be 0% and the heat source (heater) 55 is turned off. The above-described temperature control scheme is called an on/off control scheme which is disclosed by Japanese Patent No. 3746913.
In the fixing device applying the on/off control scheme, a temperature ripple with respect to a target control temperature becomes greater. Accordingly, Japanese Laid-open Patent Application No. 60-163102 discloses a PID (Proportional, Integral, and Derivative) control for optimizing multiple parameters depending on deviations of a detected temperature and the target control temperature by combining a control algorithm with proportions, integrals, and derivatives. In the PID control, the duty of turning on electricity to a heater (heat source) varies in a range of 0% through 100%.
On the other hand, an object of Japanese Laid-open Patent Application No. 2008-122757 is to make the temperature ripple smaller and stably shorten a rising time. A control technology is disclosed to vary the duty of turning on electricity to be a value calculated by using a detection result of the temperature detecting part based on a predetermined algorithm.
As described in Japanese Laid-open Patent Application No. 2008-122757, in a fixing device using the PID control alone, the temperature ripple becomes smaller. However, since the PID control frequently turns on and off the heater (heat source) even in a waiting state, energy consumption becomes greater than that of the on/off control.
On the contrary, in a case of switching from the on/off control to the PID control after printing, an overshoot becomes greater due to a temperature inside the fixing part after printing, and it takes time to assure the capability of fixing an output image.
In the above-described conventional fixing device, it is difficult to suppress the overshoot when moving to the waiting state after printing, without making the temperature ripple greater.